Aromatic molecule, oscillator strength

In some aromatic molecules that have a high degree of symmetry, i.e. with a minimum D2h symmetry (e.g. benzene, triphenylene, naphthalene, pyrene, coronene), the first singlet absorption (So —> Si) may be symmetry forbidden61 and the corresponding oscillator strength is weak. The intensities of the various forbidden vibronic bands are highly sensitive to solvent polarity (Ham effect). In polar solvents, the intensity of the 0-0 band increases at the expense of the others. 

In Figure 4 we display experimental values of r/rB and fluorescence quantum yields for a number of aromatic and heterocyclic molecules. On the whole the linear relation (3-2) is confirmed. The appreciable, though apparently nonsystematic, deviations from eq. (3-2) are not unexpected because of difficulties in measuring quantum yields, because of inaccuracies in the experimental oscillator strengths, and because of deviations expected when the ground and excited-state vibrational frequencies are different.31... 

In contrast to polyenes the aromatic molecules exhibit not only the Ti — So absorption, but also the longlived T — So emission, which gives rise to phosphorescence phenomena of rigid solvents and crystals. This is another important field of applications of spin-orbit quadratic response theory. Such calculations refer to lifetimes, transition moments, oscillator strengths and polarization directions for the radiative decay of molecular triplet states. These quantities may either be averaged over the triplet levels or refer to specific triplet spin sublevels depending on the conditions for the relevant experimental measurements. 

In molecules such as the aromatic hydrocarbons for which the most experimental data are available, it was found that the fluorescence lifetimes were much shorter than that which could be deduced from the integrated absorption intensity. The latter provides the oscillator strength f, and a radiative lifetime... 

These wavefunctions account for the attractive force between A and D leading to the formation of an EDA complex, for its increased polar character and also for the existence of charge-transfer absorption that is often observed when EDA complexes form. Low-lying excited states of A or D+ must be included in the wave function to describe additional charge-transfer bands. These bands appear in addition to the absorption bands of the molecules A and D they are usually broad and of low oscillator strengths because there is little overlap between the HOMO of the donor and the LUMO of the acceptor (Section 4.4, Equation 4.20). Many examples of the formation of EDA complexes between, for example, tetracyanoethylene and aromatic hydrocarbons are known. The... 

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